Geist Reservoir is a reservoir in the northeastern part of metropolitan Indianapolis, Indiana, USA. It was constructed in 1943 by damming Fall Creek to provide water for Indianapolis. Upon completion, Geist Reservoir was the second-largest man-made lake in Indiana, providing approximately 7,000,000,000 US gallons (2.6×1010 l; 5.8×109 imp gal) of water. The reservoir is located primarily in the northeast corner of Indianapolis and the southeast corner of Fishers, but small parts reach into the nearby towns of Lawrence, Fortville, and McCordsville.

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Geist Reservoir was named after Clarence H. Geist, a former owner of the Indianapolis Water Company who foresaw a deficit in Indianapolis's water supply and envisioned the Geist Reservoir to preemptively address the problem. Planning for the reservoir began as early as 1913, when hydraulic engineers estimated that White River and Fall Creek would not provide enough water for the increasing needs of Indianapolis. Geist gradually bought some 5,000 acres (20 km2) in Fall Creek Valley in the 1920s and 1930s, including the small town of Germantown, which today lies at the bottom of the reservoir. Although controversial, the reservoir was completed in 1943, five years after Clarence Geist's death. In the 1960s further controversy arose over plans for commercial and residential development in the area around the lake. A proposal in the 1970s to triple the size of the reservoir was defeated, and a housing boom began in the lake area.[1]

In recent years the Geist area has experienced rapid growth. The area is noted for its topography and the reservoir. In recent years the reservoir has experienced problems with algal blooms and invasive aquatic species.[2][3]

Geist Reservoir Dam is located at the lake's southern end. Fed by Fall Creek on the north, the lake overflow is directed into the creek again at the south. The earthen dam is 44 feet (13 m) high, with a length of 1,400 feet (430 m) at its crest. The reservoir capacity is 60,000 acre-feet (74,000,000 m3), although normal storage is 21,180 acre-feet (26,100,000 m3).[4]

The reservoir is mostly rather shallow (10 feet (3.0 m) or less). The area has undergone rapid development and some high-valued homes now line the reservoir's waterfront.